The present invention relates generally to a method of controlling a differential, and more specifically, to a method of automatically controlling the transfer of torque within a motor vehicle differential by monitoring the speeds of differential output shafts and comparing those speeds to a normalized target value and subsequently using specific combinations of proportional, integral and derivative (PID) control logic to transfer torque to the necessary output shaft of the differential.
Four-wheel-drive systems for the automotive, pickup truck and sport utility vehicle markets utilize a variety of mechanical, electrical and electronic systems to engage and control the driving wheels when a four-wheel-drive system activates and applies torque to a desired wheel. The amount or quantity of torque that an equipped four-wheel-drive system is capable of supplying and the time within which the system is capable of supplying or transferring torque depends upon the design of a particular four-wheel-drive system and its method of control.
Several major categories of four-wheel-drive systems are employed in vehicles. One such system consists of a permanent or full-time four-wheel-drive system. This system has no two-wheel-drive mode. Vehicles with permanent four-wheel-drive typically have a locking center differential which may split torque to the front and rear drive shafts. Electronics may or may not be employed to lock the differential to transfer torque to the front or rear driveshaft and ultimately the front or rear axle. Driving with the center differential locked makes steering more difficult or strenuous because the vehicle will tend to travel directly forward, and resist turning, especially on dry or compacted surfaces. When the center differential is open (not locked), it does not make any torque adjustments between the front and rear driveshafts.
Another type of four-wheel-drive system is on-demand four-wheel-drive. The major difference between permanent four-wheel-drive and on-demand four-wheel drive is the lack of a center differential on an on-demand system. In on-demand systems, either the front or rear axle receives all of the torque for everyday driving. Torque is transferred to the non-slipping axle when necessary, if the vehicle is so equipped. However, in most on-demand systems there is generally no way to turn off the four-wheel-drive, and likewise, the driver does not have to activate the system.
Part-time four-wheel-drive is yet another type of, four-wheel-drive system. This type of system provides torque to all four wheels upon engagement of the system by the driver. A vehicle equipped with part-time four-wheel-drive generally has an axle that receives all of the torque when the system is disengaged. A transfer case is employed to provide simultaneous torque to the front and rear axles when the system is engaged, including the normally non-driven axle, typically the front axle in many vehicles. However, there is no center differential to regulate torque between the front and rear driveshafts. Upon engagement, the front and rear axles are generally synchronized and rotate at the same speed, which improves straight line traction but makes turning the vehicle more difficult than if the speeds of the front and rear driveshafts could be altered. Because of this design, there is no way for the two axles to rotate at different speeds, in a cornering situation, for instance. Therefore, vehicle operation on loose or forgiving surfaces is necessary to permit wheel slip which helps to prevent damage to driveline components which could occur if the system is engaged and operated on dry pavement. If the vehicle is driven on dry pavement with the four-wheel-drive system activated, vehicle occupants will likely feel an awkward, vibrating rumble as a corner is turned, or even when straight line driving is attempted due to wheel speed mismatch. This vibration is caused by binding within the driveline system and may indicate impending driveline damage.
Drawbacks of the part-time four-wheel-drive system are the omission of a center differential thereby committing the front and rear axles to a matched speed and provokes the possibility of damage when the system is operated on dry pavement. Additional drawbacks of this type of system are that vehicle operators may be required to stop the vehicle to engage or disengage the system and to make the initial decision of whether road conditions warrant engaging the four-wheel-drive system. Varying road conditions may present a particular problem to drivers when deciding to engage and disengage a system. Requiring the driver to make this decision is yet another drawback of the part-time four-wheel-drive system.
Ultimately, there are automatic four-wheel-drive systems. These systems automatically determine when four-wheel-drive is necessary and transfer, as requirements necessitate, torque to the wheel(s) with the most traction, or rather, to the wheels with the least amount of wheel slip. This system requires a limited slip center differential or similar type of viscous coupling or multi-plate clutch, in a transfer case, a front and rear driveshaft, and a limited slip differential in each axle. The benefit of an automatic four-wheel-drive system is that it senses its own traction needs as the vehicle travels over any terrain, and then continuously adjusts for wheel slippage or lack of traction. This permits the driver to concentrate on driving instead of having to concern himself or herself with shifting the vehicle into or out of four-wheel-drive. However, a drawback of the automatic four-wheel-drive system is that a driver""s efforts may be hampered in severe off-road conditions. That is, because the system constantly monitors wheel slip according to the terrain, a driver may find the torque adjustment from one wheel or axle to another to be abrupt and unsettling while the vehicle negotiates such terrain. This may be especially true if the four-wheel-drive system senses a need for a torque adjustment and then lags in invoking that adjustment, causing an abrupt and unsettling shift in torque, regardless of the degree of that unsettling shift. As the speed of the torque adjustment upon wheel slip increases, the abruptness of the shift will decrease.
The above four-wheel-drive systems are all capable of providing four-wheel-drive capability in some capacity, whether it be full-time, on-demand, part-time or automatic. The automatic four-wheel-drive systems base the engagement of the system upon a change in a measurable variable. However, what all current four-wheel-drive systems lack is a method of controlling wheel slip through the utilization of a measurable variable that takes into consideration the output shaft speeds of a differential but that is also normalized with respect to vehicle speed.
What is needed then is a method of controlling the torque through a differential that does not suffer from the above four-wheel-drive system limitations. Furthermore, what is needed then is an automatic four-wheel-drive system that utilizes a method of controlling wheel slip through the invocation of a calculated target differential ratio that considers the difference between the output shaft speeds of a differential but that also normalizes the difference with respect to vehicle speed and which then continually bases torque requirements on the comparison of the actual differential ratio to the normalized target differential ratio.
In accordance with the teachings of the present invention, an automatic four-wheel-drive system is disclosed which utilizes automatic differential control logic. The invention provides a logic based system that will compare the actual difference (xcex94A) between a given differential""s output shaft speeds to an acceptable range about a target difference (xcex94T) between the differential output shaft speeds and determine, for a given differential, whether a torque bias is needed in order to decrease or prevent wheel slip from occurring.
In one preferred embodiment, the automatic differential control logic utilizes a method of logic-based control. Multiple methods of, logic based control may be adapted to the system, however, three primary methods of logic-based control are exemplified for this invention; proportional, integral, and derivative (PID) control methods. Each method of control utilizes a delta target (xcex94T) value and a delta actual (xcex94A) value. Both, xcex94T and xcex94A values are calculated using the speeds of the given output shafts, (xcfx89a and xcfx89b, for a given differential.
The equation to arrive at a xcex94T value for a given differential is:
xcex94T=(xcfx89axe2x88x92xcfx89b)/(xcfx89a+xcfx89b)
where:
xcex94T=target value for a given differential
xcfx89a=speed of a first differential output shaft
xcfx89b=speed of a second differential output shaft
The equation to arrive at a xcex94A value for a given differential is:
xcex94A=(xcfx89axe2x88x92xcfx89b)/(xcfx89a+xcfx89b)
where:
xcex94A =actual value for a given differential
xcfx891=speed of a first differential output shaft
xcfx89b=speed of a second differential output shaft
The xcex94T value is measured on a vehicle operating under a condition of no wheel slip. Therefore, for a vehicle traveling in a straight line on a hard or compacted surface with adequate traction, the measured xcex94T value will theoretically be zero and be constant. However, due to constant,:but minimal variations in steering wheel angle, disproportionate tire tread wear, disproportionate tire pressure, actual tire size, and minor variations in the road surface, the xcex94T value may actually not be zero. The non-zero xcex94T value becomes the xcex94T value for the given road surface and vehicle operating under a no-slip condition. To account for any real-world variations, an acceptable tolerance range exists around the xcex94T value. For instance, if the xcex94T is 10, and the acceptable tolerance is +/xe2x88x920.5, then a value of 10.3 falls within the tolerance range and no torque adjustment results. In other words, a value of 10.3 is considered to be a no-slip wheel situation and no torque control or adjustment results.
The xcex94A value is arrived at by the same equation but is measured during every logic loop during actual, real world, vehicle operation. The xcex94A value is then compared to the xcex94Tvalue. The absolute value of the difference between the xcex94T and the xcex94Avalue is known as the control value (CV) which a logic controller uses for torque correction and adjustment. The goal being to adjust the xcex94A value to that of the xcex94T by the amount of the CV to achieve the optimum condition of no wheel slip.
The proportional method of control utilizes the term:
(xcex94Axe2x88x92xcex94T)
and provides for a basic correction proportional to the calculated difference. Once again, a tolerance range must be provided around the xcex94T to provide for normal variation about the mean.
The integral method of control utilizes the term:       ∑          i      =      1        n    ⁢      xe2x80x83    ⁢            (                        Δ          A                -                  Δ          T                    )        i  
and makes corrections based on the accumulated error over time. This provides for an accelerated response in the event that the error is not responding to repeated proportional control logic corrections.
The final method of control logic is the derivative method of control which utilizes the term:
(xcex94Axe2x88x92xcex94T)i+1xe2x88x92(xcex94Axe2x88x92xcex94T)i
and makes corrections based on the rate of change of the error. This provides for aggressive correction when the error is increasing as opposed to when the error is decreasing or maintained at a constant level.
The present invention provides for the utilization of a logic control method such as the proportional, integral and derivative methods of logic control either individually or in particular combinations as the four-wheel-drive system operates in automatic mode. However, the vehicle operator has the ability to disengage the system from automatic mode and place the system in an xe2x80x9coffxe2x80x9d or a xe2x80x9clockxe2x80x9d mode. The xe2x80x9coffxe2x80x9d mode is utilized when the operator desires to permanently disengage the differential control system such as when a smaller diameter spare tire is installed on the vehicle. In this example, the system is prevented from continuously attempting to adjust for the smaller diameter tire. If not disabled, the system will continually attempt to adjust for the smaller tire because of different output shaft speeds, xcfx89a and xcfx89b respectively, across a differential, which normally indicate wheel slip. Alternatively, the four-wheel-drive system offers a xe2x80x9clockxe2x80x9d mode to prevent any differentiation which provides maximum traction for off-road use.
To control the output shafts of each differential, regardless of the control logic methodology, clutch packs engage and disengage. The engagement and disengagement is accomplished using a hydraulic system(s) or electric motor(s) acting on the clutch packs within each differential. Since actuation speed of the four-wheel-drive system assists the driver in maintaining control over a given terrain, the selected method, hydraulic or electric, is an important consideration when selecting the PID or other logic combinations or method of control of the system.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limited the scope of the invention.